1. Field of the Invention
The present invention relates generally to a coated polymer or plastic substrate and more particularly to a polymer substrate coated with a material such as a hard coat or other abrasion resistant material in which the refractive indices of the polymer substrate and such material are substantially matched and one or more vapor deposited optical films. The present invention also relates to a method of making a coated polymer substrate in which the refractive indices of such coated layer and the polymer substrate are substantially matched.
2. Description of the Prior Art
Various optical devices, display articles or other products exist such as windows, eyeglass lenses and protective panels for computer screens, CRTs, television sets and the like in which an object is viewed through a transparent material. If the transparent material is reflective, reflected images are formed on the material, thereby disturbing vision and creating so-called ghosts or flares. These can be tiring to the eyes, particularly when the transparent material is in the form of a protective panel or screen for a computer display or CRT where the device is commonly viewed for extended periods of time.
A number of measures have been proposed for preventing or minimizing this reflection. Where the transparent material is glass, the problem has been addressed by depositing single or multi-layer anti-reflective films on the glass substrate or lens through a variety of thin film deposition techniques. More recently for reasons of weight, safety, expense, etc., glass has been replaced by various polymer materials as the transparent material in these applications. However, attempts to apply anti-reflective films directly onto the polymer or plastic substrates have created problems with film adhesion and stability. Further, the polymer substrates commonly available are relatively soft compared to glass substrates, thereby making them susceptible to scratching and abrasion. This has led to various modifications of the polymer surface to protect it from abrasion and to make it more suitable for deposition of the anti-reflective films.
One of these modifications includes providing the polymer substrate with a hard coat of abrasion resistant material comprised of a silica-loaded, siloxane-based film which not only provides a hardened coating on the surface of the plastic substrate for protection, but also provides bonding sites for the deposited anti-reflective film. Such a hard coat applied to a plastic substrate and then cured to provide an abrasion resistant coating for such substrate is disclosed in Clark U.S. Pat. No. 3,986,997. One problem not addressed in Clark, however, is the reflection occurring at the interface between the polymer substrate and the abrasion resistant material. The amount of reflection occurring at this interface is directly related to the difference between the refractive index of the substrate and the refractive index of the hard coat. Conventional siloxane based hard coats such as those described in Clark have a relatively low refractive index on the order of 1.44 to 1.46. In direct contrast polymer substrates of acrylic polymers such as polymethylmethacrylate (refractive index n.sub.d =1.491), polystyrenes (refractive index n.sub.d =1.590), polycarbonates such as (refractive index n.sub.d =1.586) and polydiethylene glycol bisallyl carbonate (refractive index n.sub.d =1.504) and many of the various other plastic substrates disclosed in Clark have relatively high refractive indices which differ significantly from that of conventional hard coats and particularly siloxane based hard coats. Many of these other substrate materials have refractive indices of about 1.50 or greater and some of about 1.60 or greater. Applying a hard coat to a polymer substrate in which the hard coat and substrate have differing refractive indices leads to the phenomenon known as interference ripple with the distortion resulting from such ripple being directly proportional to the difference in refractive indices. Interference ripple can affect the transmission of light at various points in the optical spectrum and cause reflectivity to increase by a factor of two to three times. The problem of interference ripple is addressed in Deguchi et al. U.S. Pat. No. 4,609,267 which solves the problem by vapor depositing an anti-reflective film between the plastic substrate and the silicon dioxide (SiO.sub.2) hard coat.
Kawashima et al. U.S. Pat. No. 5,015,523 also discloses a hard coat applied to a polymer substrate. Specifically, Kawashima discloses a synthetic resin lens comprising a plastic substrate, an organic primer to promote adhesion, a siloxane based abrasion resistant hard coat and a multi-layer, anti-reflective film. No attempt is made in Kawashima, however, to match the refractive indices of the hard coat to the plastic substrate and the optical data disclosed in such patent evidence interference ripple in the Kawashima lens, thus decreasing its optical performance.
Accordingly, there is a need in the art for an effective abrasion resistant or other material applied to a plastic substrate in which the material and the substrate have substantially matching refractive indices to reduce or eliminate interference ripple as well as a method of making an optical device, display article or other product having a coated plastic transparent substrate exhibiting such properties.